Brunswick House is one of those attractive buildings that you can pass by a zillion times without ever getting a really close look. In this case, that's because it's right by the horrible roundabout system at Vauxhall bridge. Add the tube station, the bus terminus, the faint paranoia induced by the proximity of Terry Farrell's MI6 building, and the sum is a part of town that feels permanently jangled and frantic. Next to Brunswick House is the huge, hideous St George's Wharf, which makes the 1758 building seem all the smaller and more elegant and more lost.

I know someone who once went to a meeting in the MI6 building: she reported that it was like a "reverse Tardis", which seemed interesting from the outside but was dead boring once you'd got in. Brunswick House is the other way around: it looks restrained and Georgian from the outside, but once you get in, it's a riot. The building is home to the architectural salvage company Lassco, and it's full of an amazing variety of stuff: furniture of all sorts, chairs, floor lights, mirrors, chandeliers, posters, urns, metalwork. The men's toilet is, in at least two senses, funky. The main space is quirky and chaotic, an ideal example of the supreme good taste that prefers to seem messy and random. If you were to inherit a huge house and needed to furnish it in impeccably Bohemian style, you could sort yourself out here with one visit.

Brunswick House is also home to a cafe. It's run by Jackson Boxer, whose family are in the food world, too: brother Frank runs the legendary Campari Bar on top of a multi-storey car park in Peckham and dad Charlie co-runs the deli Italo, a local favourite. His grandmother is Arabella Boxer, author of a number of excellent cookbooks, including one of my all-time favourites, Book Of English Food. That has a fascinating subject: high-quality British cooking between the wars, before Elizabeth David came along and recalibrated everyone's palate towards the Med. (It's out of print, but I'm told Fig Tree is reprinting it next July: do yourself a favour.)

The short and simple way of describing Brunswick House Cafe is to say it is perfect. Jackson Boxer has made a place that sends exactly the right signals about itself: it is quirky and personal, and casual and cool, but it's very well run, too, and the underlying commitment to good food isn't casual at all. There is a feeling that the people working here enjoy what they do. That helps the customer relax and enjoy it, too.

The menu is St John-style laconic: "Courgettes and romesco", "Cauliflower, red onion and Stichelton", "Pigeon, lentils and radicchio". The simplicity leaves nowhere for the chef to hide when things go wrong, and there were a couple of points when it did: the pigeon was near-raw and the lentils tasted faintly but unmistakably burnt. Apart from that, the food ranged from good to very good. That cauliflower dish was wonderfully tangy, and the melted blue cheese made the ensemble almost gamey. A terrine of smoked Old Spot pork had masses of herby kick. Mackerel came chopped up with mustard and blackberries, a fresh, light and original take on the fish. A bowl of cockles came with mussels, haddock and a luxurious dollop of aïoli, and was so good that it made me wonder why cockles appear on menus so infrequently.

They're open during the day, too, with a more limited menu, and make dangerously good cocktails. Boxer has been very clever about building the business slowly: it's been open a while, but got an oven only this summer, and from next week it will be open on Tuesday and Wednesday evenings as well. For most young restaurateurs, I would be worried about the effect of success and rising expectations, but Boxer has done so well so far that I would bet on him to keep ahead of the curve. Restaurants are mainly about hospitality, a fact that far too many of them forget. Brunswick House Cafe doesn't, and deserves to keep on thriving. This is no one's corporate idea of a restaurant, and it sprang up from no one's business plan. Hoorah!